CHICAGO — A suspect is in custody and could face charges as soon as today in the killing of Chicago Police Cmdr. Paul Bauer.

The suspect was armed with a semi-automatic weapon and was wearing a bullet-proof vest.

Police are asking for all videos of the shooting scene -- from taxis, buildings and media witness interviews -- as they continue the investigation.

Cmdr. Paul Bauer was shot multiple times Tuesday afternoon after he spotted a man matching the radio description of an armed suspect, the city's police superintendent said. Bauer "saw the offender and engaged in an armed physical confrontation," Superintendent Eddie Johnson said. Officers had been chasing the man on foot.

Bauer, 53, had been on the police force 31 years. He's the first Chicago officer to be killed in seven-years, and the highest ranking officer to be killed in decades.

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Johnson choked back tears as he spoke to reporters outside Northwestern Memorial Hospital, where Bauer was pronounced dead.

"I just ask the citizens of this city to keep the Bauer family in their prayers," he said.

A procession of law enforcement vehicles took Bauer's body to the medical examiner's office.

Mayor Rahm Emanuel called Bauer's death "a tragic reminder of the dangerous duty the men and women of our police department accept to ensure the safety of us all."

The shooting occurred near the James R. Thompson Center. Johnson said officers initially confronted the suspect because he was acting suspiciously. Police captured the man a short time later, and a gun was recovered at the scene, he said.

It was initially reported that Bauer was off-duty, but police spokesman Anthony Guglielmi said on Twitter late Tuesday that Bauer was on-duty. He said in an email Wednesday that it was a "chaotic scene" and Bauer had been attending training at the Chicago Fire Department with other officials before the shooting.

"There was no communication from him on the radio informing dispatch that he was in pursuit of the offender or that he was even on scene, so they initially thought it was an off-duty officer until someone recognized him," Guglielmi wrote.

Some initial media reports said the suspected shooter may have been involved in the robbery of a local business, but Johnson did not provide details about what the man was doing that prompted officers to want to speak with him.

Witnesses to parts of the incident said they heard gunshots.

"I was walking down the street and I heard 'pop pop pop pop pop,'" Noreen Danko, who was on her way to her office nearby, told the Chicago Tribune. "And I said to the girl next to me, 'Is that what I think it is?' And she said, 'Yep.' I said, 'Aw geez.' And everybody is scurrying all over and I see the police head to the stairs there. There's a stairwell there. And it goes downstairs. I heard that the door is locked there. It used to be a pedway, but now, for security reasons they have it locked."

She told the newspaper that she saw officers lead the shackled suspect away and put him in a squad car, then watched as emergency workers administered CPR to Bauer, who was lying on a stretcher, before he was put inside an ambulance and rushed to the hospital.